Cyclometalated Ir(III) complexes containing N-aryl picolinamide ancillary ligands
Graphical abstract
The reaction of cyclometalated chloro-bridged iridium(III) dimer, [(ppy)2 Ir(μ-Cl)]2 (ppy – 2-phenyl pyridine) with N-aryl picolinamides (LH, LH–NO2, LH–CH3, LH–Cl, LH–F) in presence of sodium methoxide resulted in the formation of neutral heteroleptic complexes [Ir(ppy)2L] (1), [Ir(ppy)2L–NO2](2), [Ir(ppy)2L–CH3](3), [Ir(ppy)2L–Cl](4) and [Ir(ppy)2L–F] (5). 1–5 show a single emission with λmax at around 513 nm.
Highlights
► Cyclometalated Ir(III) complexes containing ancillary amide ligands have been synthesized. ► All these complexes were characterized by X-ray crystallography and ESI-HRMS studies. ► The new organometallic Ir(III) complexes showed intense green phosphorescence. ► DFT calculations on these complexes were carried out to understand their photophysical properties.
Introduction
In recent years there has been considerable research activity that has been directed towards organometallic cyclometalated Ir(III) complexes [1], [1](a), [1](b). This interest stems from both practical and academic viewpoints. Due to a strong spin-orbit coupling cyclometalated Ir(III) complexes exhibit phosphorescence with moderate to high quantum yields and because of this such complexes have been viewed as potential phosphorescent dopants in organic light-emitting diode (OLED) devices [2], [2](a), [2](b), [2](c). Other applications of these complexes are varied and include light-emitting electrochemical cells [3], [3](a), [3](b), [3](c), [3](d), electrogenerated chemiluminescence [4], [4](a), [4](b), photoinduced hydrogen production [5], photoelectrochemical solar cells [6], [6](a), [6](b) as well as phosphorescent probes/markers in biological systems [7], [7](a), [7](b), [7](c), [7](d). From an academic point of view there has been considerable interest in unraveling and understanding the precise mechanism of emission of these complexes including the details of the various excited state energy levels that are involved in this process. Theoretical studies, particularly those based on density functional theory (DFT) have been particularly useful in conjunction with experiments to make steady progress in the understanding of the photophysical processes in cyclometalated Ir(III) complexes [8], [8](a), [8](b), [8](c), [8](d).
In spite of significant progress, considerable amount of work still remains to be done before one can settle down on the ideal families of cyclometalated Ir(III) complexes that can be used in various practical devices. One of the ways of modulating the properties of the complexes vis-à-vis their molecular structures is by the choice of an appropriate cyclometalating (C^N) ligand and an ancillary ligand leading to both homoleptic [Ir(C^N)3] [9], [9](a), [9](b), [9](c) or heteroleptic [Ir(C^N)2L]n+ (n = 0, 1) complexes [10](f), [10](g), [10](h), [10](i), [10](j), [10](k), [10](l), [10], [10](a), [10](b), [10](c), [10](d), [10](e). While the cyclometalating ligand of choice mostly has been a 2-phenyl pyridine (ppy) based ligand, ancillary ligands have been varied from acetylacetonate to tetrazole. However, in spite of this attention to the ancillary ligand, surprisingly there have been very few attempts to utilize amide ligands [11], [11](a), [11](b), [11](c). One of these reports the generation of the amide linkage on the cyclometalated Ir(III) complex, in situ, by a photochemical singlet oxygen-induced process [11a]. On the other hand amide ligands have been extensively used in transition metal chemistry [12](d), [12](e), [12](f), [12](g), [12](h), [12](i), [12](j), [12](k), [12](l), [12](m), [12], [12](a), [12](b), [12](c) and are known to stabilize higher oxidation states as well as impart stability to the complexes. For example, recently, amide ligand complexes of Pd(II) have been reported. These complexes were shown to be especially stable in solution and serve as excellent catalysts in C–C bond formation reactions [13](b), [13], [13](a). In view of this we have explored the possibility of incorporating amide ligands (LH, LH–NO2, LH–CH3, LH–Cl, LH–F) to form neutral heteroleptic complexes [Ir(ppy)2L] (1), [Ir(ppy)2L–NO2](2), [Ir(ppy)2L–CH3](3), [Ir(ppy)2L–Cl](4) and [Ir(ppy)2L–] (5) where L and substituted-L are chelating (N, N) amidate ligands while the cyclometalating ligand is the phenyl pyridine ligand (Scheme 1). Synthesis, structural studies, photophysical studies, theory and electrochemistry of these complexes are reported herein.
Section snippets
Synthesis
The heteroleptic Ir(III) complexes 1–5 containing two cyclometalating 2-phenyl pyridine ligands and one deprotonated N-aryl picolinamide ligand were prepared by a sequential two step synthetic procedure (Scheme 1). The cyclometalated chloro-bridged iridium(III) dimer, [(ppy)2 Ir(μ-Cl)]2 was prepared by using the Nonoyama protocol; subsequent reaction of this iridium(III) dimer with sodium methoxide followed by reaction with N-aryl picolinamides afforded the neutral, monomeric, heteroleptic
Conclusions
In conclusion, we have prepared and characterized neutral cyclometalated Ir(III) complexes containing N-aryl picolinamide ligand, [Ir(ppy)2L] (1), [Ir(ppy)2L–NO2] (2) [Ir(ppy)2L–CH3] (3) [Ir(ppy)2L–Cl] (4) and [Ir(ppy)2L–F] (5) where L and substituted-L are chelating (N, N) amidate ligands while the cyclometalating ligand is the 2-phenyl pyridine ligand. The use of amide ligands in cyclometalated Ir(III) complexes is sparse and the current study reveals the effectiveness of these ligands in the
General
All procedures involving Ir(III) complexes were carried out under a nitrogen gas atmosphere using Schlenk techniques. 2-Phenyl pyridine was purchased from Aldrich Chemical Company, USA and was used as such without further purification. Iridium chloride was purchased from Arora Matthey, Kolkata, India. Other chemicals were purchased from S. D. Fine-Chemicals, Mumbai, India. Solvents were purified by conventional methods and were freshly distilled under nitrogen atmosphere prior to use according
Acknowledgement
We thank the Department of Science and Technology (DST), India for financial support. B.M. and P.B. are thankful to CSIR, New Delhi, India for Fellowship. We also thank Dr. Pratik Sen (IIT Kanpur) for valuable suggestions and R. Suriya Narayanan for TGA studies.
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